1. Field of the Invention
The present invention relates to hydroprocessing methods and reactors and, more particularly, to multiple bed downflow reactors for the catalytic hydroprocessing of hydrocarbons.
2. Description of the Prior Art
The reaction of hydrocarbons, particularly heavier petroleum feedstocks such as distillates, lubricants, heavy oil fractions, residuum, etc., usually in the presence of a catalyst and elevated temperatures and pressures, is known as hydroprocessing. Typical hydroprocessing processes include hydrodesulfurization, hydrodenitrification, hydroisomerization, hydrodemetallation, hydrocracking, hydrogenation, and the like. A hydroprocessing reactor may have two or more catalytic beds containing the same or different catalysts, depending upon the intended utility of the beds. Therefore, depending upon the catalysts, two or more of these processes may be carried on in the same reactor.
In a typical hydroprocessing reactor, for example for desulfurization, a vertical reactor is divided into one or more catalyst-containing zones. Liquid feed is introduced at the top of the reactor together with the hydrogen gas and the liquid feed in co-current contact with the hydrogen gas passes through a catalyst bed containing a desulfurization catalyst. The more labile feed components react quickly, diluting the hydrogen with H.sub.2 S (which can inhibit the desired reactions) and with light hydrocarbon gases. The liquid leaving the bottom of the reactor is in contact with gas containing the highest amount of H.sub.2 S and light hydrocarbon gases and a relatively low hydrogen partial pressure, which limits the extent of sulfur removal.
In another hydrodesulfurization reactor, liquid feed is introduced at the top of the reactor and flows downwardly through the desulfurization catalyst bed. Hydrogen gas is directed under pressure into the bottom of the reactor and flows upwardly through the catalyst bed in countercurrent contact with the downflowing liquid feed. In this arrangement, exiting liquid at the bottom of the reactor is in contact with the fresh incoming hydrogen gas, which contributes to very high reaction rates and desulfurization yields. However, countercurrent flow in a packed bed containing small catalyst particles is problematic and is to be avoided where possible.
In still another hydrodesulfurization processes, the reactor includes two or more vertically stacked catalyst beds. Hydrogen gas is fed co-currently with the liquid feed to an upper desulfurization zone in the presence of a desulfurization catalyst. Liquid effluent from the first zone flows downwardly to a lower desulfurization zone wherein the liquid effluent is contacted with a countercurrent flow of hydrogen in the presence of a desulfurization catalyst. This combined co-current-countercurrent process mitigates some of the reaction rate and yield disadvantages of conventional co-current process but suffers all of the disadvantages of countercurrent flow in packed beds containing small catalyst particles.
Currently a large number of hydrodesulfurization and other hydroprocessing reaction systems experience extremely unfavorable kinetic conditions or utilize countercurrent flow, with its inherent disadvantages, to improve the kinetics performance of the processes. Accordingly, a hydroprocessing technique that would improve overall kinetics efficiency while maintaining conventional co-current downflow through the catalyst beds would be desirable.